Insulin resistance represents a rapidly expanding health care burden in the United States, contributing to the pathogenesis of a number of cardiometabolic disorders. Recent evidence has emerged that microbes resident in the human intestine represent a key transmissible environmental factor contributing to insulin resistance and associated cardiometabolic disease. However, mechanisms by which gut microbial-derived factors signal to the host to promote insulin resistance are largely unknown. We have recently discovered a meta-organismal pathway where nutrients present in high fat foods (phosphatidylcholine, choline, and L-carnitine) can be metabolized by the gut microbial enzymes to generate trimethylamine (TMA), which is then further metabolized by the host enzyme flavin-containing monooxygenase 3 (FMO3) to produce trimethylamine-N- oxide (TMAO). Preliminary studies here demonstrate that the TMAO pathway is linked to insulin resistance and the development of type 2 diabetes in humans and animal models. Moreover, we show that pharmacologic inhibition of TMAO production confers striking protection against insulin resistance in mice. Mechanistically, we have found that host metabolic reprogramming driven by the glucocorticoid receptor (GR) requires direct transcriptional regulation of FMO3. Collectively, our preliminary data have led us to propose the following central hypothesis: The gut microbial metabolite TMAO is a GR-sensitive mediator of host stress responses that promote insulin resistance. The specific aims are: Aim 1. Testing the hypothesis that the gut microbial TMAO pathway directly impacts susceptibility for high fat diet-driven insulin resistance; and Aim 2. To determine whether transcriptional regulation of the host TMAO-producing enzyme FMO3 is necessary for GR-driven immunosuppression and metabolic reprogramming. We anticipate our studies to reveal new molecular mechanisms linking gut microbe-derived factors to insulin resistance and associated cardiometabolic disorder, which will ultimately be leveraged into to the first ever gut microbe-targeted therapeutics for the treatment of type 2 diabetes.